The present invention relates to a graphic display apparatus having a boundary detection target region designating circuit necessary for boundary detection of a blanking out processing region and having a boundary detection memory in which all the two-dimensional figures corresponding to boundaries of figure blanking out processing are written.
A graphic display apparatus having a frame memory for storing figures has a figure processing function for blanking out a region bound by an arbitrary closed figure. The graphic display apparatus of this type has a memory (boundary detection memory) for temporarily storing figures in addition to the frame memory. In blanking out processing, all the two-dimensional figures (closed figures) to be boundaries of regions (target regions for figure blanking out processing) are temporarily written in the boundary detection memory. Thereafter, an operation for detecting a boundary is performed while read accessing all the areas of the boundary detection memory in a scan line direction, and a given blanked-out figure is written in the frame memory in accordance with the boundary detection result. The blanked-out figure, as shown in FIG. 1, is displayed on a screen in accordance with the content of the frame memory. Note that FIG. 1 illustrates a state wherein regions surrounded by closed FIGS. 11 and 12 are blanked out.
Referring to FIG. 1, rectangle 13 indicated by a broken line is a minimum rectangle including all the boundaries of the blanked-out regions. The region of rectangle 13 is normally only a small fraction of display region 14. Therefore, in order to detect the boundary of such a small region, it is time-consuming to read out all the regions of possible display region 14.